1. Field of the Invention
The present invention relates to an image fixing apparatus mounted in an image forming apparatus, such as a copier or a printer.
2. Description of the Related Art
Thermal-roller image fixing apparatuses and on-demand image fitting apparatuses, well known conventional apparatuses, are mounted in image forming apparatuses, such as electrophotographic copiers and laser beam printers.
A thermal-roller image fixing apparatus includes a heating device and a pair of juxtaposed rollers (a fixing roller and a pressurizing roller), and when a recording sheet, such as paper, on which toner has been deposited is passed through a nip (a fixed portion), the point at which the rollers contact each other, the toner is melted and fixed to the sheet.
Of the pair of rollers, the fixing roller, which contacts the obverse surface of a sheet (the toner carrying side), is a hollow cylinder, the surface of which is composed of a material that exhibits a superior releasing property and is heated by an internally mounted halogen heater, the heating device, to melt the toner. The pressurizing roller, which contacts the rear face of the sheet, has an elastic layer formed on a core metal, and is appropriately pressed against the toner layer.
Ideally, applied heat melts all toner and fixes it to the surface of a sheet. However, when in the cold offset state there is, unmelted toner, or when in the hot offset state melted toner is heated excessively, toner (hereinafter referred to as stained toner) is electrostatically offset to the fixing roller, or this stained toner is fixed to the surface of either the fixing roller or the pressurizing roller, whichever roller has a surface that has an inferior releasing property.
When the releasing property of the fixing roller is inferior to that of the pressurizing roller, the stained toner is fixed to the fixing roller.
During image forming, however, the fixing roller is constantly heated to a toner melting temperature. Therefore, the stained toner is in the melted state, and since this toner is transferred to a sheet by being mixed with toner on the surface of the next sheet, a state wherein the fixing roller will be continuously contaminated is seldom present. However, there are conditions wherein stained toner is present on the surface of the fixing roller, and the staining of images may occur.
When the releasing property of the pressurizing roller is inferior to that of the fixing roller, stained toner that is offset to the fixing roller is moved to the pressurizing roller. The temperature of the pressurizing roller is lower than that of the fixing roller, and stained toner is not always present on the pressurizing roller in a state wherein it is completely melted.
Further, since the toner image on a sheet does not contact the pressurizing roller, the removal of stained toner with a toner image does not occur, and once the roller is contaminated, contamination toner continues to be accumulated.
Further, when the pressurizing roller continues to be contaminated with toner, the releasing property of the pressurizing roller is reduced. Thus, a sheet (especially an OHT film sheet) would stick to the pressurizing roller, or accumulated stained toner would be transferred at one time to the reverse side of the sheet, which is stained.
An on-demand image fixing apparatus enables a quick start and power reduction, compared with a thermal roll image fixing apparatus. Using the on-demand system, the thermal energy of the fixing apparatus is reduced by using a ceramic heater and a thin film, such as a polyimido film, so that a quick start and power reduction are implemented (Japanese Patent Application Laid-open Nos. 63-313182, 2-157878, 4-44075 to 4-44083 and 4-204980 to 4-204984).
Since the on-demand fixing apparatus requiring only a small amount of thermal energy and has a temperature response property, the fixing apparatus does not need to be warmed in advance, precise temperature control is available, and the supply of power to the fixing apparatus can be halted, except during the paper feeding time.
In the on-demand image fixing apparatus, however, under the above temperature control, the pressurizing roller is not heated except during the paper feeding time, so that the temperature is not easily raised, compared with the thermal roller system. The temperature of the pressurizing roller is raised only to 120xc2x0 C., at most.
Therefore, stained toner that is offset to the fixing film and is shifted to the pressurizing roller tends substantially to be attached to the pressurizing roller, instead of being melted thereon.
When the feeding occurs of a recording material containing CaCO3, the CaCO3 is mixed with the offset toner, and the viscosity of the toner is reduced. Therefore, it is difficult to remove toner from the surface of the pressurizing roller, and contamination and deterioration of the pressurizing roller occurs.
To resolve this problem, in an image forming apparatus that has been proposed and has been put into use U.S. Pat. No. 6,175,699, fixing is halted after printing has been completed, and the heating member of the image forming apparatus is used to raise the temperature at a fixing nip until it is equal to or higher than the toner softening point (hereinafter referred to as xe2x80x9cpost-heating controlsxe2x80x9d).
During this process, heat is produced by the heating member after the fixing apparatus has been halted, following the completion of the printing, toner attached to and accumulated on the pressurizing roller is melted at the fixing nip, and when the next printing sequence begins, the softened toner is shifted to the fixing roller or to the fixing film.
Then, when during the next fixing process a sheet is passed through the fixing nip, toner adhering to the fixing roller or the fixing film is transferred to the surface of the sheet, and the sheet, together with the added toner, is discharged.
The offset toner at this time has been accumulated through several to several tens of sheets were fed through. Therefore, the volume of the toner is too small to be recognized, and accordingly, the volume of the toner transferred to a sheet is too small to be noticed.
According to what is proposed above, on the outer face of the pressurizing roller, the portion that is stopped at the fixing nip after the printing is ended differs each time the pressurizing roller is halted. Therefore, by repeating the printing process, the contamination across the entire outer face of the pressurizing roller can be removed. As a result, the accumulation of contamination, such as toner, on the pressurizing roller can be prevented.
It has been found, however, that even when toner attached to the pressurizing roller is transferred to the fixing roller or to fixing film under post-heating control, toner is again transferred to the pressurizing roller during a preparation period for the initiation of the next fixing process. It has also been found that regardless of whether an image fixing apparatus is a heater roller type, a film type or an electromagnetic induction type, this toner re-transfer phenomenon easily occurs with an apparatus in which substantial heat is not applied to the fixing roller and the pressurizing roller is cool while the apparatus is on stand-by waiting for the transmission of a print signal.
To resolve the above shortcomings, it is one objective of the present invention to provide an image fixing apparatus that can remove offset toner.
It is another objective of the present invention to provide an image fixing apparatus that consumes little power and that can prevent a pressurizing roller from being contaminated.
It is an additional objective of the present invention to provide an image fixing apparatus that can prevent a pressurizing roller from being contaminated and that can provide a satisfactory fixing function.
It is a further objective of the present invention to provide an image fixing apparatus comprising:
a heating member;
a back-up roller cooperating with the heating member to form a nip therebetween for conveying the recording material;
control means, for controlling the generation of heat by the heating member and
wherein the control means is capable of effecting first control for permitting the heating member to generate heat in a state where an image fixing process is completed and the back-up roller is stopped, and second control for maintaining the heating member at a target temperature, differing from a fixing temperature, during a warm-up period the heating member starts to generate heat until the temperature of the heating member reaches the fixing temperature.
It is a still further objective of the present invention to provide an image fixing apparatus comprising:
a heating member;
a back-up roller cooperating with the heating member to form a nip therebetween for conveying the recording material;
control means for controlling generation of heat by the heating member,
wherein the control means is capable of effecting control for maintaining the heating member at a target temperature, differing from a fixing temperature, during a warm-up period from when the heating member starts to generate heat until the temperature of the heating member reaches the fixing temperature, and
wherein the target temperature is a temperature such that, during a period where the heating member is maintained at the target temperature, 70% or more of the toner, which is remained on the heating member and is not transferred to the back-up roller, is present.
It is one more objective of the present invention to provide an image fixing apparatus comprising:
a heating member;
a back-up roller cooperating with the heating member to form a nip therebetween for conveying the recording material; and
control means for controlling generation of heat by the heating member,
wherein the control means is capable of effecting control for maintaining the heating member at a target temperature, differing from a fixing temperature, during a warm-up period from when the heating member starts to generate heat until the temperature of the heating member reaches the fixing temperature, and
wherein 20% or more of the toner adhered to the heating member before a start of the warm-up period remains on the heating member at an end of the warm-up period.
The other objectives of the invention will become apparent during the course of the following detailed explanation, given while referring to the accompanying drawings.